Antennae were first developed towards the end of the past century, when James C. Maxwell in 1864 postulated the fundamental laws of electromagnetism. Heinrich Hertz may be attributed in 1886 with the invention of the first antenna by which transmission in air of electromagnetic waves was demonstrated. In the mid forties were shown the fundamental restrictions of antennae as regards the reduction of their size relative to wavelength, and at the start of the sixties the first frequency-independent antennae appeared. At that time helixes, spirals, logo periodic groupings, cones and structures defined solely by angles were proposed for construction of wide band antennae.
In 1995 were introduced the fractal or multifractal type antennae (Spanish Patent number 9501019), which due to their geometry presented a multifrequency behavior and in certain cases a small size. Later were introduced multitriangular antennae (Spanish Patent number 9800954) which operated simultaneously in bands GSM 900 and GSM 1800.
The antennae described in the present patent have their origin in fractal and multitriangular type antennae, but solve several problems of a practical nature which limit the behavior of said antennae and reduce their applicability in real environments.
From a scientific standpoint strictly fractal antennae are impossible, as fractal objects are a mathematical abstraction which include an infinite number of elements. It is possible to generate antennae with a form based on said fractal objects, incorporating a finite number of iterations. The performance of such antennae is limited to the specific geometry of each one. For example, the position of the bands and their relative spacing is related to fractal geometry and it is not always possible, viable or economic to design the antennae maintaining its fractal appearance and at the same time placing the bands at the correct area of the radioelectric spectrum. To begin, truncation implies a clear example of the limitations brought about by using a real fractal type antenna which attempts to approximate the theoretical behavior of an ideal fractal antenna. Said effect breaks the behavior of the ideal fractal structure in the lower band, displacing it from its theoretical position relative to the other bands and in short requiring a too large size for the antenna which hinders practical applications.
In addition to such practical problems, it is not always possible to alter the fractal structure to present the level of impedance of radiation diagram which is suited to the requirements of each application. Due to these reasons, it is often necessary to leave the fractal geometry and resort to other types of geometries which offer a greater flexibility as regards the position of frequency bands of the antennae, adaptation levels and impedances, polarization and radiation diagrams.
Multitriangular structures (Spanish Patent number 9800954) were an example of non-fractal structures with a geometry designed such that the antennae could be used in base stations of GSM and DCS cellular telephony. Antennae described in said patent consisted of three triangles joined only at their vertices, of a size adequate for use in bands 890 MHz–960 MHz and 1710 MHz–1880 MHz. This was a specific solution for a specific environment which did not provide the flexibility and versatility required to deal with other antennae designs for other environments.
Multilevel antennae solve the operational limitations of fractal and multitriangular antennae. Their geometry is much more flexible, rich and varied, allowing operation of the antenna from two to many more bands, as well as providing a greater versatility as regards diagrams, band positions and impedance levels, to name a few examples. Although they are not fractal, multilevel antennae are characterised in that they comprise a number of elements which may be distinguished in the overall structure. Precisely because they clearly show several levels of detail (that of the overall structure and that of the individual elements which make it up), antennae provide a multiband behavior and/or a small size. The origin of their name also lies in said property.
The present invention consists of an antenna whose radiating element is characterised by its geometrical shape, which basically comprises several polygons or polyhedrons of the same type. That is, it comprises for example triangles, squares, pentagons, hexagons or even circles and ellipses as a limiting case of a polygon with a large number of sides, as well as tetrahedra, hexahedra, prisms, dodecahedra, etc. coupled to each other electrically (either through at least one point of contact o through a small separation providing a capacitive coupling) and grouped in structures of a higher level such that in the body of the antenna can be identified the polygonal or polyhedral elements which it comprises. In turn, structures generated in this manner can be grouped in higher order structures in a manner similar to the basic elements, and so on until reaching as many levels as the antenna designer desires.
Its designation as multilevel antenna is precisely due to the fact that in the body of the antenna can be identified at least two levels of detail: that of the overall structure and that of the majority of the elements (polygons or polyhedrons) which make it up. This is achieved by ensuring that the area of contact or intersection (if it exists) between the majority of the elements forming the antenna is only a fraction of the perimeter or surrounding area of said polygons or polyhedrons.
A particular property of multilevel antennae is that their radioelectric behavior can be similar in several frequency bands. Antenna input parameters (impedance and radiation diagram) remain similar for several frequency bands (that is, the antenna has the same level of adaptation or standing wave relationship in each different band), and often the antenna presents almost identical radiation diagrams at different frequencies. This is due precisely to the multilevel structure of the antenna, that is, to the fact that it remains possible to identify in the antenna the majority of basic elements (same type polygons or polyhedrons) which make it up. The number of frequency bands is proportional to the number of scales or sizes of the polygonal elements or similar sets in which they are grouped contained in the geometry of the main radiating element.
In addition to their multiband behavior, multilevel structure antennae usually have a smaller than usual size as compared to other antennae of a simpler structure. (Such as those consisting of a single polygon or polyhedron). This is because the path followed by the electric current on the multilevel structure is longer and more winding than in a simple geometry, due to the empty spaces between the various polygon or polyhedron elements. Said empty spaces force a given path for the current (which must circumvent said spaces) which travels a greater distance and therefore resonates at a lower frequency. Additionally, its edge-rich and discontinuity-rich structure simplifies the radiation process, relatively increasing the radiation resistance of the antenna and reducing the quality factor Q, i.e. increasing its bandwidth.
Thus, the main characteristic of multilevel antennae are the following:                A multilevel geometry comprising polygon or polyhedron of the same class, electromagnetically coupled and grouped to form a larger structure. In multilevel geometry most of these elements are clearly visible as their area of contact, intersection or interconnection (if these exist) with other elements is always less than 50% of their perimeter.        The radioelectric behavior resulting from the geometry: multilevel antennae can present a multiband behavior (identical or similar for several frequency bands) and/or operate at a reduced frequency, which allows to reduce their size.        
In specialized literature it is already possible to find descriptions of certain antennae designs which allow to cover a few bands. However, in these designs the multiband behavior is achieved by grouping several single band antennae or by incorporating reactive elements in the antennae (concentrated elements as inductors or capacitors or their integrated versions such as posts or notches) which force the apparition of new resonance frequencies. Multilevel antennae on the contrary base their behavior on their particular geometry, offering a greater flexibility to the antenna designer as to the number of bands (proportional to the number of levels of detail), position, relative spacing and width, and thereby offer better and more varied characteristics for the final product.
A multilevel structure can be used in any known antenna configuration. As a nonlimiting example can be cited: dipoles, monopoles, patch or microstrip antennae, coplanar antennae, reflector antennae, wound antennae or even antenna arrays. Manufacturing techniques are also not characteristic of multilevel antennae as the best suited technique may be used for each structure or application. For example: printing on dielectric substrate by photolithography (printed circuit technique); dieing on metal plate, repulsion on dielectric, etc.
Publication WO 97/06578 discloses a fractal antenna, which has nothing to do with a multilevel antenna being both geometries essentially different.